I found the problem.
I don't know but after installing the EPi 7.5 my gadget started to get references of files in version 7.5 ( Episerver.dll, EpiServer.data.dll ) and I have tried to install my nupack on a version 7. It started to make sense when I tried to install in Epi 7.5
It makes no sense because I started my development just with version 7.0 installed on my environment. By default my references were to a GAC.
I just redefined the references to use the files of version 7.